1. Field of the Invention
The invention relates to a signal transmission method according to the principle of quadrature/amplitude modulation which digital data occurring at a time interval T are formed into a transmit signal with the aid of pulse shapers having a given unit pulse response r(t) in a transmitter, the transmit signal is transmitted by means of a carrier oscillation via a channel having a channel unit pulse response h(t) when an additive white noise w(t) is superimposed on it, so that a receive signal y(t) is present at a receiver and the receive signal y(t) is preprocessed by channel-matched filters in the receiver.
2. Discussion of Background
Rugged digital signal transmission methods have a central significance in the digitisation of mobile radio sets. These methods can be divided into several classes. It is especially the method of quadrature/amplitude modulation (QAM) which is to be considered at this point. The basic principles of such methods are described in detail, for example, in the book by J. G. Proakis, "Digital Communications", New York, McGraw-Hill, 1983.
One problem which must be solved in order to be able to implement such a signal transmission method is the identification of the channel via which signals are transmitted from a transmitter to a receiver. The quality of such an identification, also called estimation, essentially determines the extent to which a receiver can operate without errors. Particularly in the case of moble radio systems, the channel changes with time so that it is not possible to identify the channel once and for all. Instead, it is necessary to embed a method which reestimates the channel from time to time in the signal transmission method. In the literature, such methods are treated under the title of system identification and are described in detail, for example, in the book by P. Eykhoff, "System Identification, Parameter and State Estimation", London, Wiley, 1974.
Usually, channels of great length are discussed, that is to say, channels the unit pulse response of which decays slowly in comparison with a period given by the time interval of the data occurring in the transmitter. The channel is then determined only with a coarse resolution. However, it has now been found that it is of significance for an optimum detection of the transmitted data in the receiver, also to estimate short channels with a good resolution. As an example, the simple case of a channel having a fast, short echo shall be mentioned. However, the known methods fail in such cases, the numeric methods become unstable.